1. Technical Field of the Invention
The present invention relates to the decoding of radiofrequency transmission channels conveying coded digital information.
The invention thus applies advantageously to digital terrestrial television, that is to say using signals transmitted between television antennas, as defined, for example, in the European DVB-T (Digital Video Broadcasting-terrestrial) or in the North-American ATSC (Advisory Committee on Advanced Television Service Digital TV) specification, or to digital cable television, as defined, for example, in the European DVB-C (Digital Video Broadcasting-cable) or in the North-American ITU-J83 Annex A/B/C/D (International Telecommunication Union) specification, all these telebroadcasts being based on the MPEG transmission standards, and use for example to convey information, quadrature digital modulation, or else modulation of the COFDM type according to terminology well known to the person skilled in the art.
The invention also relates in particular to tuners, and also to demodulators and to the actual channel decoding processing.
2. Description of Related Art
The television signals received at an input of a tuner of a receiver are composed of the entire set of channels transmitted that lie in the 45 MHz-860 MHz frequency band. The objective of the tuner is to select the desired channel and to output a baseband signal on the in-phase path (I path) and on the quadrature path (Q path). This signal is thereafter converted into a digital signal and demodulated. The channel decoding processing thereafter also comprises a block which distinguishes, typically by means of majority logic, zeros from ones, then performs all the error correction, that is to say typically a Viterbi decoding, the interleaving, Reed-Solomon decoding and deshuffling. The channel decoding device outputs packets that are decoded in a conventional manner in a source decoding device in accordance with the MPEG standards so as to regenerate the initial audio and video signals transmitted via antennas or via the cable.
Current studies are aimed at researching ever more integrated solutions for the embodiment of digital terrestrial or cable television receivers. However, this research hits technological difficulties related to the embodiment of the means making it possible, on the one hand, to correctly discern a channel from the entire set of channels present, and, on the other hand, to correctly discriminate the signal from noise. Specifically, whereas in digital satellite television, all the channels have nearly the same power, this is not the case in particular in digital terrestrial television. Thus, it is for example possible to have adjacent channels exhibiting a very high power with respect to the desired channel, for example a difference in power of the order of 40 dB. It is therefore necessary to be able to ensure rejection of 40 dB on the adjacent channels. Moreover, the specification imposes a signal/noise ratio of the order of 30 dB. Hence, in the worst case, it is therefore essential for it to be possible to bring the level of an adjacent channel to 70 dB below its input level. This results in extremely significant constraints on the filters, this being extremely constraining in respect of integrated solutions.
There is accordingly a need for a solution to this problem.
There is also a need for a totally integrated solution, given the characteristics of digital television signals, in particular terrestrial ones, exhibiting more significant rejection constraints than digital cable television signals.